target/arm: Implement MVE VCADD

target/arm: Implement MVE VCADD

Implement the MVE VCADD insn.  Note that here the size bit is the
opposite sense to the other 2-operand fp insns.

We don't check for the sz == 1 && Qd == Qm UNPREDICTABLE case,
because that would mean we can't use the DO_2OP_FP macro in
translate-mve.c.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>